It's not that he didn't know they're Americans, he just temporarily forgot. Just like when you go sleep over at someone else's place but it takes a moment to remember you're not at home when you wake up the next morning.

Okay...
Consider this; be in England, go to some British high security place (we have lots of them so just pick one), be like "you're British, you're not going to shank me" but then you have to be told they're Scottish. I mean, you're already in Britain so you should already know. Scottish are British, too

He forgot what Americans are like. He knew they were Americans, he knows what Americans are like. But he forgot the second part of that. Saying they are Americans was a reminder that we are fond of guns. It was not a reminder that Americans inhabit America. I am honestly surprised at how many people seem not to have gotten this.

Perhaps.
But this is man who was probably there the time when England burned down the whitehouse on Canada's behalf, maybe he was confused, maybe that tiny section of history out of the millions of years on millions of planets than he knows, maybe he thought they were Canadians.